Polyester resins (e.g., copolyester elastomers, polyalkylene terephthalates, and the like) are known to degrade when exposed to ultraviolet light. The degradative effects of ultraviolet light on polyester resins is typically evidenced by a distinct color change in the resin. That is, when exposed to ultraviolet light, the color of polyester resins usually changes from a white appearance to a grey or yellow appearance after time. Moreover, this color change usually occurs rapidly with the rate of color change in response to exposure to ultraviolet light thereafter decreasing over time.
Recently, polyester resins (particularly polyester elastomers and polyester resins blended with impact modifiers) have been used in automotive applications, i.e., as molded parts on the interior and/or exterior of an automobile. Needless to say, any color change of the original molded polyester resin is unacceptable when used as a part for an automobile, particularly a part that is visible. To counteract the tendency of polyester resins to degrade upon exposure to ultraviolet light, a variety of UV-light stabilization systems have been proposed.
In U.S. Pat. No. 4,185,003, for example, thermoplastic copolyetherester elastomers are stabilized against heat and light aging by incorporating into the copolyetherester an effective concentration of a phenolic antioxidant and a hindered amine photostabilizer. Japanese patent Publication No. 75/91652 discloses the use of a number of hindered piperidine type photostabilizers in combination with phenolic antioxidants in copolyetheresters. However, according to U.S. Pat. No. 4,185,003, while improvements to photostabilization of the copolyetheresters is improved when the teaching of this Japanese Publication is followed, the heat-aging behavior is much poorer when the photostabilizer is present compared to performance in the absence of the photostabilizer (see, column 1, lines 30-45 of U.S. Pat. No. 4,185,003).
U.S. Pat. No. 4,136,090 suggests that copolyetheresters may be stabilized against oxidative degradation due to exposure to heat and light by incorporating into the polymer an effective concentration of a phenolic antioxidant and copolymerized hindered amine photostabilizer.
In U.S. Pat. No. 4,340,718, polyester materials, particularly copolyesters, are rendered less susceptible to weathering by incorporating into the polyester resin a dimethyl and diethyl ester of p-methoxybenzylidenemalonic acid having monofunctional terminal ester forming groups or a difunctional comonomer.
U.S. Pat. Nos. 4,355,155 and 4,405,749 each disclose segmented thermoplastic copolyester elastomers which may be stabilized against heat. For example, in U.S. Pat. No. 4,355,155 such stabilizers may include phenols and their derivatives, amines and their derivatives, compounds containing both hydroxyl and amine groups, hydroxyazines, oximes, polymeric phenolic esters and salts of multivalent metals in which the metal is in its lower state (see column 3, lines 46-52), while in U.S. Pat. No. 4,405,749, a particular triazine-based anti-oxidant (i.e., 3,5-di-tert-butyl-4- hydroxyhydrocinnamic acid triester with 1,3,5-tris-(2-hydroxyethyl)-s-triazine-2,4,6-(1H, 3H, 5H) trione) is disclosed as having superior thermal stability. Each of the compositions of these prior issued U.S. patents moreover mention that additional stabilization against ultraviolet light may be obtained by compounding the copolyetheresters with various UV absorbers, such as substituted benzophenones or benzotriazoles (see, column 4, lines 1-3 of U.S. Pat. No. 4,355,155; and column 4, lines 10-13 of U.S. Pat. No. 4,405,749).
Blends of polybutylene terephthalate and a segmented thermoplastic copolyester elastomer may also be stabilized against heat or ultraviolet light via the addition of various stabilizers. For example, in U.S. Pat. No. 3,907,926, anitoxidants and amide stabilizers may be incorporated into the PBT/polyester elastomer blends (see column 6, line 55 bridging column 7, line 25), while U.S. Pat. No. 4,469,851 suggests the phenol derivatives (including 3,5-di-tert-butyl-4- hydroxyhydrocinnamic acid triester with 1,3,5-tris-(2-hydroxyethyl)-s-triazine-2,4,6-(1H, 3H, 5H) trione) may be incorporated into the PBT/polyester elastomer blends for UV-light stabilization functions.
A three-way stabilization system for polyester elastomers is also known (see DuPont Elastomers Laboratory Technical Notes for Hytrel.RTM. polyester elastomer, Mar. 2, 1977). In essence this three-way system consists of equal parts of a phenolic antioxidant (tetrakis (methylene-3-(3,5-di-tert- butyl-4-hydroxy-phenyl propionate) methane), a benzotriazole absorber (i.e., 2-3(3',5'-di-t-butyl-2'-hydroxyphenyl)-5-chlorobenzotriazole) and a hindered piperdine type compound (i.e., bis(2,2,6,6-tetramethyl-4-piperidinyl)sebacate). The presence of the latter piperdine type compound is, however, suggested to be an indispensable component.
Surprisingly, by employing the additives as will be described below, polyester molding compositions are rendered resistant to weathering, and more particularly, are rendered resistant to degradation by UV light.
Broadly, the present invention is based upon the unexpected discovery that when certain triazine-based antioxidants and benzophenone UV-absorbers are incorporated into polyester molding compositions, synergistic effects in terms of UV-light stabilization properties of the resulting resins ensue. In particular, it has been found that when the triazine-based antioxidants and benzophenones (to be described in greater detail below) are incorporated into polyester resins, the color difference of the resin when exposed to ultraviolet light is significantly lower as compared to other known UV-light stabilized polyester resins commercially available and other known UV-light stabilization systems. For example, it has been found that the polyester resin compositions of this invention exhibit a desirably low color difference, as calculated in CIELab units under illuminant "D-65" according to ASTM Standard D-2244, of less than about 10.0, when samples are exposed in a Xenon arc weather-ometer according to SAE J1885 for 315 Kj/sq.m.